Paulo Monteiro of the University of California, Berkeley, studied the results of tests conducted by the U.S. Bureau of Reclamation for more than 40 years. The bureau subjected concrete cylinders of varying water-to-cement ratios and cement compositions to a mild acid bath to test when they would fail. Because the liquid stone does not dry to become strong but rather solidifies from a chemical reaction between the cement and water, concrete is susceptible to sulfate attack. These salt solutions actually penetrate the concrete and change the chemical reaction between the cement and water both during and after the solidification process, ultimately causing structural failure.
Current techniques for fending off sulfates include cement mixtures low in tricalcium aluminate and with a low water-to-cement ratio, because it makes for a less reactive and less porous concrete. But the 40-year experiment proved that the water-to-cement ratio had little impact on the life span of the concrete. Rather it is the physical components of the concrete that proves the determining factor. Monteiro even worked out specific formulas that can more accurately determine a given concrete's endurance depending on its constituent parts.
"These models can be used to predict the remaining life of existing structures," Monteiro writes in the paper presenting the finding in this week's Proceedings of the National Academy of Sciences. "Such knowledge also can be used to determine which combinations of mixes are more optimal for use in aggressive environments, leading the way to more robust structures less prone to environmental degradation." In other words, our constructions might finally last longer than Roman ones.